1. Field of the Invention
The present invention relates to a method for purifying an organometallic compound in a reusable state from an exhaust gas after thin film formation in the CVD method using an organometallic compound as a raw material.
2. Description of the Related Art
Since the chemical vapor deposition method (hereinafter referred to as CVD method) has the advantages that it can easily prepare uniform thin films and it is superior in step coverage, the method is one of the thin film-forming technologies generally employed for production of film electrodes in semiconductor devices. The CVD method is considered to become a major process for preparation of film electrodes in the future because the method can provide further densification required of recent circuits and electronic parts.
The CVD method is a preparation method of a thin film of a metal or a metal oxide, which involves vaporizing a raw metal compound to transport the same to the surface of a substrate, reacting the transported raw material particle on the substrate to accumulate as a metal or a metal oxide to form a thin film. As a raw material of metal compound, in particular, organometallic compounds, which have low melting points and are easy to handle, are used (the CVD method using an organometallic compound is hereinafter referred to as MOCVD).
Incidentally, the production costs of thin films by the MOCVD method depend on the prices of organometallic compounds which are raw materials and the ratio of the amount of the organometallic compounds consumed in the reaction to the amount of the compound introduced on the surface of a substrate, i.e., the utilization ratio, and the utilization ratio in the conventional MOCVD method is 10% or lower, which means most of the introduced source gas is actually disposed as an exhaust gas. Therefore, the production costs of thin films with such low utilization ratios are strongly influenced by the prices of organometallic compounds.
In this connection, the prices of organometallic compounds are generally high because their synthesis require multiple steps. For example, copper itself is not expensive as a metal but the price significantly increases when an organometallic compound thereof is synthesized. Therefore, it is considered that the high costs of forming thin films according to the conventional MOCVD method with low utilization ratios cannot be avoided due to the high prices of organometallic compounds.
Specifically, thin films of precious metals such as ruthenium and iridium are being employed recently for higher performance of film electrodes and since precious metals are trace metals and are expensive themselves, organometallic compounds thereof are significantly expensive. Therefore, the production costs of thin films of precious metals according to the conventional MOCVD method are expected to be extremely high.
As described earlier, the organometallic compounds are easy to handle and capable of forming thin films efficiently and they are expected to be increasingly employed. Therefore, if demand for the organometallic compounds increases, the conventional MOCVD method with low utilization ratios has the disadvantages relating to the production costs of thin films and about deletion of resources due to a huge lose of the materials.
The applicant developed a recycling method of organometallic compounds for MOCVD wherein an unreacted organometallic compound component is extracted from a used raw material which was conventionally discarded and then purified to a reusable state, as a method for reducing the production costs of thin films and avoiding depletion of resources, and filed a patent application for an MOCVD thin film production process incorporating this recycling technology (application No. 2000-96359). This recycling technology involves purification and extraction of organometallic compounds by passing the raw material after the thin film formation, for example, through a cold trap to cool and condense to recover the same in a liquid state and distilling the recovered material under appropriate conditions. This technology can avoid wasting organometallic compounds and hence reduce the production costs of thin films.
Although the main point of the above-mentioned MOCVD thin film production process resides in the recycling technology, the recycling technology needs some improvements. The recycled organometallic compounds should have substantially as high purity as the virgin material in order to produce thin films with comparable properties to those made from the virgin material. It is also required that as much organometallic compounds as possible be extracted from a used raw material in order to reduce the production costs of thin films. In particular, the purity is extremely important for organometallic compounds for use as a raw material for MOCVD and if an organometallic compound containing a trace amount of impurities is used as a raw material for MOCVD, the purity of the thin film also decreases, which may affect the electric characteristics and influence the morphology of the film.
The present invention has been made under these situations and proposes an improved recycling method of organometallic compounds in which an unreacted organometallic compound is extracted from a used raw material which has undergone the CVD steps and recycled, wherein the organometallic compound of high purity is extracted efficiently.
The inventor has examined improvements in order to improve the above-mentioned recycling technology. As a result, the inventor has conceived the idea that it is necessary to assess the possibility that reaction products formed by the thin film formation reaction and its side reaction are present in a used raw material, and their properties and effects in order to complete a superior recycling technology.
In view of the effects of reaction products here, if the reaction products can be removed by normal reparation methods such as distillation, they do not signify aside from a reduction in the yield. In this connection, the inventor has thought the reaction products already identified in the conventional researches do not matter. The reason is that the impurities identified in these conventional researches include low molecular weight compounds such as water, carbon dioxide, aldehydes, and formic acid, which are originated from decomposed organometallic compounds due to leaving of metal atoms and these low molecular weight compounds have significantly different properties from those of the organometallic compounds from which they are to be separated and thus can be easily separated by normal separation methods such as distillation. Therefore, it is considered if the actual CVD thin film formation process produces only these compounds, the above-mentioned conventional recycling technology can sufficiently cope with the compounds.
However, the inventor has examined a used raw material after the thin film formation reaction in detail, and found that the actual CVD thin film formation process produces the above-mentioned low molecular weight compounds as reaction products, but also produces other reaction products due to unreported side reactions.
In this connection, a case is taken as an example for illustration in which a ruthenium thin film is produced with diethylruthenocene represented by Formula 1, which is recently receiving attention as a raw material for ruthenium thin films, and its used raw material is recycled. 
Diethylruthenocene, represented by Formula 1, is a compound composed of a ruthenium atom as a metal atom and two ethyl derivatives of cyclopentadiene coordinating to the metal as ligands. The CVD thin film formation process with diethylruthenocene involves a decomposition reaction of diethylruthenocene on a substrate to accumulate ruthenium.
The inventor has fully examined the composition of the used raw material after the thin film formation with diethylruthenocene and found the used raw material contains predominantly unreacted diethylruthenocene but the following two kinds of side reaction products are also present.
The first kind of side reaction products include, for example, organic ruthenium compounds (the following formula) with an alcohol group, a carbonyl group, an ether group, and the like to which the ethyl group (a substituent of the ligand) on the cyclopentadiene ring is oxidized. It is considered that these compounds are formed by reactions between unreacted diethylruthenocene which has not been involved in the thin film formation reaction and oxygen, which is generally added as a reactant gas when thin films are formed by using relatively stable organometallic compounds such as diethylruthenocene. 
The second kind of side reaction product is vinylcyclopentadienyl(ethylcyclopentadienyl)ruthenium represented by the following formula formed by another side reaction in the thin film formation reaction in which unreacted diethylruthenocene is dehydrogenated. An ethyl group of diethylruthenocene is dehydrogenated into a vinyl group to form vinylcyclopentadienyl(ethylcyclopentadienyl)ruthenium. Although the factors responsible for the dehydrogenation reaction are not clear, the inventor believes that the dehydrogenation reaction occurs by the catalytic action of ruthenium atom leaving from diethylruthenocene due to decomposition of the thin film formation. It is thought the dehydrogenation reaction is more likely to occur especially in the thin film formation with organometallic compounds having functional group on the ligands coordinating to metal atoms. 
It is thought the thin film formation and side reactions attributable to the organometallic compounds decomposed in the film formation reaction as well as oxidation reactions and dehydrogenation reactions toward the unreacted organometallic compounds occur in the thin film formation process in the actual MOCVD method as in the case of diethylruthenocene. Therefore, the inventor has examined a possibility for removal of oxides and dehydrogenated products of these unreacted organometallic compounds and its approach. As a result, the inventor has conceived the understanding the products generated by the oxidation reaction can be removed by normal separation methods such as distillation, but the reaction products formed by the dehydrogenation reaction have similar properties such as molecular weights and boiling points to those of the organometallic compounds to be extracted, and it is impossible to separate those components by normal separation methods such as distillation. The inventor has reached the conclusion that if the problem about these irremovable dehydrogenated products is left unsolved, the yield of recycled organometallic compounds and the purity decrease and, therefore, the electric characteristics and morphology of the thin film produced therefrom also deteriorate.
Consequently, the inventor has intensively investigated to search for methods for removing these dehydrogenated products and found the following two approaches to the removal.
The first method for removing the hydrogenated products is a reforming treatment in which the reaction products are hydrogenated or reduced to the original organometallic compounds followed by purification of the organometallic compounds.
Accordingly, the first invention of the present application is a recycling method of the organometallic compounds for MOCVD in which unreacted organometallic compounds are extracted from a used raw material which has undergone the thin film formation process, wherein the unreacted organometallic compounds are extracted after the used raw material is reformed by contacting the same with a hydrogenation catalyst or a reducing agent.
The first method converts the dehydrogenated products, which are contained in the used raw material and are difficult to separate, into the original organometallic compounds. Therefore, according to the present invention, the dehydrogenated products which are impurities disappear due to the reforming treatment, which improves the purity of the organometallic compounds to be purified later. The organometallic compounds thus obtained can be used as a raw material for MOCVD again. Because the first method converts the dehydrogenated products which are impurities into the original organometallic compounds, the concentration of the original organometallic compounds in the used raw material also increases. This also improves the yield of the organometallic compounds in the purification.
Although those generally known as hydrogenation catalysts or reducing agents can be used as the hydrogenation catalysts or reducing agents employed in the method here, either platinum catalyst, palladium catalyst, ruthenium catalyst, or Raney nickel catalyst is preferably used as the hydrogenation catalyst. As the reducing agent, sodium borohydride (NaBH4), lithium aluminium hydride (LiAlH4) calcium hydride (CaH2), dimethylamineborane (C2H7NBH4), trimethylamineborane (C3H9NBH4), or hydrazine (NH2NH2) is preferably used.
The second method for removing the dehydrogenated products of the organometallic compounds is a method in which the organometallic compounds are purified after the dehydrogenated products are subjected to a reforming treatment to convert the same to other compounds removable by separation methods such as distillation. The method for converting the dehydrogenated products to other compounds removable by separation methods such as distillation include addition reactions of halogen or hydrogen halide, addition reactions of water, addition reactions of alkenes, and addition reactions of dienes (Diels-Alder reaction) toward the dehydrogenated products.
The second invention of the present application is a recycling method of the organometallic compounds for MOCVD in which unreacted organometallic compounds are extracted from a used raw material which has undergone the thin film formation process, wherein the unreacted organometallic compounds are extracted after the used raw material is subjected to a reforming treatment by contacting the same with either a halogen, a hydrogen halide, an inorganic acid, an alkene, or a diene.
According to the second method, the dehydrogenated products of the organometallic compounds, which are contained in the used raw material and are difficult to separate, are subjected to halogen addition or hydration to change to other compounds having increased molecular weights or different boiling points. As a result, it is possible to remove the converted dehydrogenated products in the subsequent purification process of the organometallic compounds. Therefore, the second method can purify the organometallic compounds of high purity, which can be used as a raw material for MOCVD again.
Incidentally, the compounds for the reforming treatment of the used raw material include halogens such as chlorine, bromine, and iodine, hydrogen halide such as hydrogen chloride, hydrogen bromide, and hydrogen iodide, and inorganic acids such as hydrochloric acid and sulfuric acid. The alkene includes maleic anhydride and the diene includes 2-methyl-1,3-butadiene.
The recycling method according to the present invention can prevent the dehydrogenated products of the unreacted organometallic compounds, which are impossible to separate and remove, from remaining in the organometallic compounds to be recycled and allows the organometallic compounds of high purity to be recycled.
According to the inventor, it is more preferable the unused organometallic compounds are extracted after the used raw material is subjected to a decoloring treatment by contacting the same with a decoloring agent comprising either activated carbon, silica, or activated clay in order for the organometallic compounds of high purity to be recycled.
The decoloring treatment is set in view of the fact that the raw material for MOCVD may undergo a phenomenon in which it colors through the thin film formation process as in the case, for example, where diethylruthenocene, which is originally pale yellow, turns brown through the thin film formation process. The reason for the coloring is not clear but, according to the inventor, there is a problem that the used raw material thus colored is purified by a normal purification method to obtain the organometallic compounds having the same color and the decoloring treatment is to solve this problem. As the decoloring agent for decoloring, activated clay is employed in the present invention. This selection is made based on the finding that activated carbon is effective for decoloring the used raw material for MOCVD to which the present invention is directed by the trials by the inventor although it is known that activated carbon, silica, and activated clay generally adsorb a variety of materials and have deodorizing and decoloring effects.
Incidentally, the decoloring treatment may be conducted either before or after the above-mentioned reforming treatment. The decoloring treatment may be omitted when a precious metal catalyst such as platinum catalyst, palladium catalyst, or ruthenium catalyst is employed as a hydrogenation catalyst in the above-mentioned first recycling method. These precious metal catalysts generally have activated carbon as a support, which naturally functions as a support and also as a decoloring agent and therefore the reforming and decoloring treatments are simultaneously conducted when the precious metal catalyst is employed. However, a platinum catalyst and the like are used both as a decoloring agent and a reforming agent, the life of the hydrogenation activity may be shortened due to adherence of poisons attributed to coloring components to the catalyst. Therefore, when this situation is anticipated, it may be decided in view of the economy of the process whether the decoloring and reforming treatments are simultaneously conducted by a hydrogenation catalyst or the decoloring treatment by activated carbon and the reforming treatment by a hydrogenation catalyst are conducted separately.
Incidentally, the present invention is characterized in that the used raw material which has undergone the thin film formation process is subjected to the reforming treatment and/or the decoloring treatment before purification of the organometallic compounds. Therefore, there are no specific limitations to methods for recovering the used raw material before these treatments and methods for extracting and purifying the organometallic compounds from the used raw material after these treatments.
However, a step to remove oxygen from the used raw material which has undergone the thin film formation process is preferably included before the reforming treatment and/or the decoloring treatment in order to improve the yield of the organometallic compounds to be recycled. As mentioned above, oxygen may be added as a reactant gas in the MOCVD method and the oxydation reaction of the unreacted organometallic compounds may occur in the thin film formation reaction as mentioned above as well as in the recovering step when the organometallic compounds are recovered in the presence of oxygen, leading to a decrease in the yield. The methods for removing oxygen from the used raw material include contacting the used raw material with a deoxidizing agent such as silicagel.
As for the method for recovering the used raw material, the used raw material is preferably cooled to a liquid in view of easy handling in the subsequent steps, which is subsequently subjected to the reforming treatment and/or the decoloring treatment. Although the cooling conditions depend on the properties of the organometallic compounds, non-subliming organometallic compounds are preferably cooled to a temperature 30xc2x0 C. lower than the boiling points and subliming organometallic compounds are preferably cooled to a temperature 30xc2x0 C. lower than the melting points in order to fully liquefy the used raw material emitted in gaseous form. Specific approaches to the method include, for example, a method in which a cold trap is placed downstream from the reaction chamber and the used raw material is cooled in the cold trap to recover.
Furthermore, as the purification method of the organometallic compounds after the reforming treatment and the decoloring treatment, the organometallic compounds are preferably separated by distillation from the liquefied used raw material. Since the organometallic compounds have generally low melting points and low boiling points and undergo phase change at relatively low temperatures, it is possible to directly separate the organometallic compounds to high purity by distillation. Another reason is that distillation does not require a complicated apparatus and is a relatively easy purification method.
Incidentally, another preferable form as the method for purifying the organometallic compounds after the reforming treatment and the decoloring treatment is a method in which the used raw material after the reforming treatment and/or the decoloring treatment is subjected to column chromatography. This method enables separation and purification of highly pure organometallic compounds with an appropriate column packing. Moreover, the purification method using column chromatography is applied to the liquefied used raw material but also effective for the used raw material in gaseous form. Therefore one of the advantages of using column chromatography as a purification method is that the used raw material in gaseous form which has undergone the thin film formation process is directly purified without liquefying the same in the recovering step. Incidentally, either silicagel, octadecylsilane, alumina, porous polymers, graphite carbon, or zeolite is preferably used as a column packing for column chromatography.
Lastly, there is no specific limitation to the organometallic compounds for use in the present invention. Therefore, organic compounds of a variety of metals such as copper, indium, tantalum, tungsten, molybdenum, titanium, rhenium, barium, and strontium, which are generally used as raw materials for thin films conventionally, can be applied to the recycling method of the present invention. The prices of these metals are low themselves but corresponding organometallic compounds are significantly high in price and the recycling method can reduce the costs of thin film formation of these metals or metal oxides.
Moreover, the present invention is especially useful for recycling organometallic compounds comprising precious metals such as silver, gold, platinum, palladium, ruthenium, rhodium, iridium, and osmium in view of a recent increase in demand for thin films of precious metals and high prices of organometallic compounds of precious metals.